BEIRUT, April 17 (Reuters) - International mediator Lakhdar Brahimi said on Thursday that a deal between trapped fighters and civilians in Homs city and the Syrian authorities had broken down, as government forces appeared close to retaking the besieged opposition area.

Homs, a religiously-mixed city, was the scene of early protests against President Bashar al-Assad in 2011 and has become a symbol of the destructive nature of Syria's civil war, with many of its neighbourhoods levelled by army shells.

Hundreds of people remain trapped in the old part of the city, surrounded by government forces and pro-Assad militia. A deal agreed at peace talks in Geneva this year allowed some civilians to leave but further negotiations broke down following heavy fighting this week.

"It is a matter of deep regret that negotiations were brutally stopped and violence is now rife again when a comprehensive agreement seemed close at hand," Brahimi said in a statement.

"It is alarming that Homs, whose people have suffered so much throughout these past three years is again the theatre of death and destruction."

In recent months, government forces have recaptured several rebel-held areas and border towns, closing off rebel supply routes from Lebanon and securing the main highway leading north from Damascus towards central Syria, Homs and the Mediterranean.

The opposition National Coalition, a political body in exile, warned of a massacre if Assad's forces were to push through into the small pocket of rebel-held Homs.

"We warn the international community of a potential massacre in Homs. The Old City has been besieged by regime forces for 676 days," it said in a statement.

Monzer Akbik, spokesman for the group, said it was "critical that the eyes of the world remain fixed on Homs at this crucial time. The regime has reduced what was the soul of the revolution to rubble and ruin."

More than 150,000 people have been killed in the civil war, which began as peaceful protests against Assad's rule, a third of them civilians, according to the anti-Assad Syrian Observatory for Human Rights. Millions have fled the country. (Reporting by Oliver Holmes in Beirut, Dasha Afanasieva in Istanbul and Stephanie Nebehay in Geneva; Editing by Robin Pomeroy)